Polyisocyanate used as a raw material of polyurethane has been industrially produced by reacting polyamine with carbonyl chloride for an isocyanate reaction.
Such polyisocyanate has been industrially produced by continuously supplying a polyamine solution including polyamine, a carbonyl chloride solution containing carbonyl chloride, and a solvent to a reactor, reacting them so as to generate polyisocyanate, and continuously distilling the generated polyisocyanate (see the following Patent Document 1).
When polyamine and carbonyl chloride are directly supplied to a reactor so as to be reacted, a reaction temperature rises so that a side reaction easily generates. Thus, a following technique to obtain polyisocyanate effectively has been known, that is, the technique by the steps of circulating a portion of a reaction solution distilled from a reactor in the reactor again, supplying polyamine and carbonyl chloride to its circulatory line, reacting them at low temperature by a cooling device interposed in the circulatory line, and reacting a resulting reaction solution at high temperature (see the following Patent Document 2).
Patent Document 1: Japanese Unexamined Patent Publication No. 2004-035492)
Patent Document 2: Japanese Unexamined Patent Publication No. 57-165358)